1. Field of the Invention
The present invention relates to a quantity-of-light adjusting apparatus for an image pickup apparatus or the like using an ND (neutral-density) filter, a control method for the quantity-of-light adjusting apparatus, and a computer program product providing a control program for the quantity-of-light adjusting apparatus.
2. Description of Related Art
FIG. 21 is a block diagram showing the arrangement of an image pickup apparatus, such as a video camera, serving as a first example of prior art.
In FIG. 21, there are illustrated an image forming lens 501, an ND filter 502 for attenuating light, an iris 503 for adjusting the quantity of light, an image sensor 504, such as a CCD, a CDS/AGC circuit (correlated-double-sampling/automatic-gain-control circuit) 505, and a route 506 leading to a video signal processing circuit for forming a television signal.
There are further illustrated a luminance signal detecting circuit 507 for detecting a luminance signal from a video signal outputted from the CDS/AGC circuit 505, an iris control signal computing circuit 508 for forming a control signal for controlling the iris 503 according to luminance information outputted from the luminance signal detecting circuit 507, a driver 509 for driving the iris 503, and an ND-filter switching lever 510 for switching between the insertion and detachment of the ND filter 502.
Next, the operation of the image pickup apparatus shown in FIG. 21 is described.
As a luminance signal for use in controlling an exposure, there is used a luminance signal, including a high-frequency component, included in a video signal outputted from the CDS/AGC circuit 505. The luminance signal is detected by the luminance signal detecting circuit 507 and is, then, sent to the iris control signal computing circuit 508. The iris control signal computing circuit 508 computes an iris control signal by comparing the luminance signal with a predetermined reference value (a correct exposure level), in such a way as to make always constant the luminance signal detected by the luminance signal detecting circuit 507.
For example, with the comparison between the luminance signal detected by the luminance signal detecting circuit 507 and the above-mentioned reference value, if the luminance signal is not less than the reference value, such a control signal as to cause the iris 503 to operate in the closing direction is generated, and, if the luminance signal is less than the reference value, such a control signal as to cause the iris 503 to operate in the opening direction is generated. The control signal as generated is supplied to the iris 503 through the driver 509. The response at which the exposure is controlled by the iris 503 (the amount of change of exposure per unit time) is set always constant. If such a response is either too fast or too slow, the user would be given an unnatural feeling, so that the tuning of the response is said to be a difficult point.
Next, the operation for controlling the iris 503 is described with reference to the flow chart of FIG. 22.
First, a luminance signal, including a high-frequency component, included in a video signal outputted from the CDS/AGC circuit 505 is detected (step S601). Then, the detected luminance signal is compared with a predetermined reference value (a correct exposure level). If the luminance signal is not less than the reference value, such a control signal as to cause the iris 503 to operate in the closing direction is computed, and, if the luminance signal is less than the reference value, such a control signal as to cause the iris 503 to operate in the opening direction is computed (step S602). Then, the computed control signal is supplied to the iris 503 through the driver 509 (step S603).
Next, the ND filter 502 is described.
The user is allowed to operate the ND-filter switching lever 510 so as to insert and detach the ND filter 502 into and from an optical path of the lens 501, thereby selecting the use or nonuse of the ND filter 502. The basic usage of the ND filter 502 resides in that, with the ND filter 502 inserted when the luminance of an object is high, it is possible to prevent the so-called small-aperture diffraction phenomenon due to the iris 503, and, with the ND filter 502 detached when the luminance of an object is low, it is possible to increase the sensitivity of the image pickup operation.
Next, a second example of prior art is described.
Heretofore, a variety of proposals have been made about the white balance control in image pickup apparatuses, such as video cameras. In the following, the second example of prior art is described with regard to the white balance control having such a presetting function as an outdoor mode (5600K mode) or an indoor mode (3200K mode).
FIG. 23 is a block diagram showing an interchangeable-lens-type image pickup system according to the second example of prior art.
Referring to FIG. 23, the interchangeable-lens-type image pickup system is composed of a lens unit 113 and a camera body 114 on which the lens unit 113 is detachably mounted.
The lens unit 113 includes an image forming lens 101, an ND filter 102 for attenuating light, an iris 103 for adjusting the quantity of light, an ND-filter switching lever 112 for inserting and detaching the ND filter 102, and a lens microcomputer 111.
The camera body 114 includes an image sensor 104, such as a CCD, a CDS/AGC circuit 105, an A/D converter 106 for converting an analog video signal into a digital video signal, a camera signal processing circuit 107, a route 108 for outputting a television signal formed by the camera signal processing circuit 107, a camera microcomputer 109, a communication line 110 for communication between the camera microcomputer 109 and the lens microcomputer 111, and a WB mode selection switch 115 for allowing the user to select the WB (white balance) mode, such as the outdoor mode or the indoor mode.
The camera signal processing circuit 107 includes a luminance/chrominance signal forming circuit 120 for converting the digital video signal outputted from the A/D converter 106 into a high-frequency component YH of the luminance signal, a low-frequency component YL of the luminance signal and chrominance signals R and B, a gain control circuit 121 for the red signal R, a gain control circuit 122 for the blue signal B, a color-difference signal forming circuit 123 for forming color-difference signals R-Y and B-Y from the chrominance signals R′ and B′ gain-controlled by the gain control circuits 121 and 122 and the low-frequency component YL of the luminance signal, and an encoder 124 for forming the television signal from the color-difference signals R-Y and B-Y and the high-frequency component YH of the luminance signal.
Next, the operation of the interchangeable-lens-type image pickup system shown in FIG. 23 is described.
When the lens unit 113 is mounted on the camera body 114, electric power is supplied from the camera body 114 to the lens unit 113. Further, the user is allowed to operate the ND-filter switching lever 112 so as to insert and detach the ND filter 102 into and from an optical path of the lens 101, thereby selecting the use or nonuse of the ND filter 102.
Optical image light from an object is made to pass through the lens 101, and is then attenuated by the ND filter 102. Then, after being adjusted by the iris 103 so as to make a correct exposure, the optical image light is imaged on the image sensor 104. A video signal obtained by the photoelectric conversion at the image sensor 104 is subjected to the noise-removing and gain-control process at the CDS/AGC circuit 105, and is then converted into a digital video signal by the A/D converter 106. The digital video signal is sent to the luminance/chrominance signal forming circuit 120 included in the camera signal processing circuit 107.
At the luminance/chrominance signal forming circuit 120, the high-frequency component YH of the luminance signal, the low-frequency component YL of the luminance signal and the chrominance signals R and B are formed from the digital video signal. The red signal R and the blue signal B are respectively inputted to the gain control circuits 121 and 122, and are amplified there according to white balance control signals outputted from a gain control signal output circuit 125, so as to be outputted as chrominance signals R′ and B′, respectively.
The chrominance signals R′ and B′ are supplied, together with the low-frequency component YL of the luminance signal, to the color-difference signal forming circuit 123, where the color-difference signals R-Y and B-Y are formed. The color-difference signals R-Y and B-Y are supplied, together with the high-frequency component YH of the luminance signal, to the encoder 124, where the standard television signal is formed and outputted.
The camera microcomputer 109 reads the switching state of the WB mode selection switch 115 to make a check to find if the WB mode selection switch 115 is set to the outdoor mode (5600K mode) or the indoor mode (3200K mode) or the automatic mode. The camera microcomputer 109 forms, according to the mode as set, gain control signals for R gain and B gain which are beforehand stored in the camera microcomputer 109, and outputs the gain control signals to the gain control signal output circuit 125.
Next, the operation of the interchangeable-lens-type image pickup system shown in FIG. 23 is further described with reference to the flow charts of FIGS. 24 and 25. Referring to FIG. 24, the lens microcomputer 111 detects the ON/OFF-state of the ND-filter switching lever 112 to make a check to find if the ND filter 102 is in an ON-state (the state in which the ND filter 102 is inserted into the optical path of the lens 101) or in an OFF-state (the state in which the ND filter 102 is detached from the optical path of the lens 101) (step S801). If it is found that the ND filter 102 is in the ON-state, the lens microcomputer 111 sets an ND-filter-ON status flag (step S802), and transmits the thus-set ND-filter-ON status flag to the camera microcomputer 109 (step S804). On the other hand, if it is found that the ND filter 102 is in the OFF-state, the lens microcomputer 111 clears the ND-filter-ON status flag (step S803), and transmits the thus-cleared ND-filter-ON status flag to the camera microcomputer 109 (step S804).
Subsequently, referring to FIG. 25, the camera microcomputer 109 receives the ND-filter-ON status flag from the lens microcomputer 111 (step S901). Then, the camera microcomputer 109 reads the switching state of the WB mode selection switch 115 to make a check to find if the WB (white balance) mode is the outdoor mode (5600K mode) (step S902). If a result of the check made at step S902 indicates “YES”, the camera microcomputer 109 forms gain control signals for R gain and B gain which are beforehand determined correspondingly with the outdoor mode (step S903), and outputs the gain control signals for R gain and B gain to the gain control signal output circuit 125 (step S907).
If the result of the check made at step S902 indicates “NO”, the camera microcomputer 109 reads the switching state of the WB mode selection switch 115 to make a check to find if the WB mode is the indoor mode (3200K mode) (step S904). If a result of the check made at step S904 indicates “YES”, the camera microcomputer 109 forms gain control signals for R gain and B gain which are beforehand determined correspondingly with the indoor mode (step S905), and outputs the gain control signals for R gain and B gain to the gain control signal output circuit 125 (step S907).
If the result of the check made at step S904 indicates “NO”, the camera microcomputer 109 judges the WB mode selection switch 115 to be in the automatic mode, and computes gain control values for R gain and B gain for the automatic mode (step S906). Then, the camera microcomputer 109 outputs, to the gain control signal output circuit 125, the computed gain control values as gain control signals for R gain and B gain (step S907).
Next, the WB mode is briefly described with reference to FIGS. 26(a) to 26(d).
The outdoor mode is a WB (white balance) mode which is recommended to be used in picking up an image outdoors. Sunlight in the outdoors has generally high color temperature and is greatly bluish. Therefore, in the camera signal processing circuit 107, the R gain is controlled to become high and the B gain is controlled to become low, so that it becomes possible to reproduce the color close to that looked at even in the outdoors.
This arrangement can be verified by using a vector scope shown in FIG. 26(a). The vector scope shown in FIG. 26(a) is obtained in a case where, with all surfaces of a light box of color temperature of 5600K made white, an image pickup operation is performed in the 5600K mode. As is understandable from this diagram, a color exists at the center of the vector scope. This means that it is possible to recognize an object to be white.
The indoor mode is a WB mode which is recommended to be used in picking up an image indoors. Illumination light in the indoors has generally low color temperature and is greatly reddish. Therefore, in the camera signal processing circuit 107, the R gain is controlled to become low and the B gain is controlled to become high, so that it becomes possible to reproduce the color close to that looked at even in the indoors.
This arrangement can be verified by using a vector scope shown in FIG. 26(c). The vector scope shown in FIG. 26(c) is obtained in a case where, with all surfaces of a light box of color temperature of 3200K made white, an image pickup operation is performed in the 3200K mode. As is understandable from this diagram, a color exists at the center of the vector scope. This means that it is possible to recognize an object to be white.
It is preferable that the ND filter 102 is colorless. However, in actuality, when the ND filter 102 is mass-produced, the variance of spectral characteristics occurs, so that ND filters tinged with various colors, such as that tinged with red or tinged with blue, would be mass-produced.
In a case where such a tinged ND filter is used, if the ND filter 102 is brought into the ON-state, for example, in the so-called white state in which a bright point exists at the center of the vector scope as shown in FIG. 26(a), a picked-up image would be tinged due to the tinge of the ND filter 102 as shown in FIG. 26(b). As is understandable from this diagram, the position of the bright point on the vector scope deviates from the center of the vector scope. This means that the picked-up image is tinged with orange.
In the case of FIG. 26(d), which is similar to the case of FIG. 26(b), if the ND filter 102 is brought into the ON-state in such a state as shown in FIG. 26(c), a picked-up image would be tinged, so that the position of a bright point on the vector scope deviates from the center of the vector scope toward the direction of orange, in a manner similar to the case of FIG. 26(b). While FIGS. 26(b) and 26(d) show a case where the ND filter is tinged with orange, as an example, the ND filter may be tinged with red or blue.
Next, a third example of prior art is described.
A conventional image pickup apparatus, such as a video camera, according to the third example of prior art is described with reference to FIG. 27 and FIGS. 28(a) to 28(c).
Referring to FIG. 27, the image pickup apparatus according to the third example of prior art includes a photographic lens 1, an iris 2, an image sensor 3, a CDS/AGC circuit 4, an A/D converter 5, a digital signal processing circuit 6, a D/A converter 7, a microcomputer 8 for performing a logical arithmetic operation, an IG meter 9, a Hall element 10, an iris encoder 11 and an iris driving circuit 12.
Next, the operation of the image pickup apparatus according to the third example of prior art is described.
An object image made incident on the image sensor 3 by the photographic lens 1 is photoelectrically converted into an electrical signal by the image sensor 3. The electrical signal is correlated-double-sampled and amplified to a suitable level by the CDS/AGC circuit 4. The thus-processed electrical signal is converted into a digital signal by the A/D converter 5. The digital signal is converted into a standardized video signal, such as that of the NTSC system, by the digital signal processing circuit 6. The video signal is then converted into an analog signal by the D/A converter 7 and is externally outputted.
On the other hand, the rotational position of the IG meter 9, which is arranged to drive the iris 2 in the opening direction and in the closing direction, is magnetically detected by the Hall element 10. A result of detection of the rotational position of the IG meter 10 is amplified and offset-controlled to a suitable level by the iris encoder 11 and is then taken in the microcomputer 8 as data after A/D conversion.
During the above-mentioned process, the microcomputer 8 reads information on the video signal level from the digital signal processing circuit 6 and information on the opening-and-closing state of the iris 2 from the iris encoder 11. Then, the microcomputer 8 computes a control signal for the iris 3 and outputs the control signal to the iris driving circuit 12 in such a way as to make the video signal level small if it is too large or in such a way as to make the video signal level large if it is too small. The iris driving circuit 12 drives the IG meter 9 according to the control signal.
In the above-described process, an iris mechanism composed of the iris 2, the IG meter 9 and the Hall element 10 operates in such a way as to make the brightness of an object image formed on the image sensor 3 constant. Here, in a case where the brightness of an object is extremely high, the aperture diameter of the iris 2 becomes very small. In this case, the sharpness of an object image formed on the image sensor 3 is sometimes lost due to the diffraction phenomenon of light.
In order to prevent this problem, in general video cameras, an ND filter, serving as an achromatic light-attenuating filter, is made to be inserted into the space between the photographic lens 1 and the iris 2, so that the aperture diameter of the iris 2 is prevented from becoming smaller than a given value.
In many cases, if the iris 2 is of a general twin-blade type, the ND filter is arranged integrally with an iris blade, i.e., is stuck to a part of the iris blade.
An example of the iris mechanism using the ND filter is shown in FIGS. 28(a) to 28(c).
Referring to FIGS. 28(a) to 28(c), the iris mechanism includes iris blades 21 and 22, an ND filter 23 stuck to a part of the iris blade 21, and an IG meter 9, and a rotor 91 mounted on the rotary shaft of the IG meter 9.
Assuming that, in the beginning, the iris 2 is in a fully-open state as shown in FIG. 28(a) and then begins to close, when the IG meter 9 is driven to cause the rotor 91 to rotate around the rotary shaft of the IG meter 9, the aperture of the iris mechanism comes to take such a shape as shown in FIG. 28(b) and, at the time of completion of closing of the iris mechanism, comes to take such a shape as shown FIG. 28(c).
As the aperture diameter of the iris 2 becomes smaller in such a progress as shown in FIG. 28(a) to FIG. 28(c), the ratio in area of the ND filter 23 to the aperture shape of the iris 2 becomes greater. When the aperture diameter of the iris 2 becomes small up to a certain degree, the ND filter comes to cover the whole area of the aperture shape of the iris 2.
In such an iris mechanism as described above in which the ND filter 23 exists at a part of the aperture shape of the iris, no problem arises when an object image formed on the image pickup plane is in an in-focus state. However, when the object image is in an out-of-focus state, or when the object image is an image other than an aimed object, for example, a background, since the focusing state of the object image is uneven in many cases, the shape of a circle of confusion for the object image would become very irregular.
In general, when the object image is in an out-of-focus state, or when the object image is the one which is necessarily in an out-of-focus state, such as a background, it is preferable, from the viewpoint of observation, that the shape of a circle of confusion for the object image is close to a true circle. The shape other than the one close to a true circle results in that the so-called taste of blurring is not good.
In the case of low-priced general video cameras, such a bad taste of blurring almost does not give rise to any problem. However, in the case of high-grade video cameras in which importance is attached to the image-quality function in some degree, it is a relatively important subject to improve the taste of blurring. Accordingly, in some of the high-grade video cameras, in order to improve the taste of blurring, the ND filter is not arranged integrally with the iris mechanism. Instead, there is provided a mechanism for inserting the ND filter from outside, or the camera body is provided with an external switch, such as a switching lever as described in the first and second examples of prior art, to enable the user to insert and detach the ND filter according to necessity.
In the image pickup apparatus according to the first example of prior art as shown in FIG. 21, there is such a first problem that, since the ND filter causes the transmission factor for the quantity of light in the image pickup optical system to greatly decrease, when the ND filter is switched from the ON-state to the OFF-state or when the ND filter is switched from the OFF-state to the ON-state, the quantity of light made incident on the image sensor varies greatly before and after the switching of the ND filter, so that the variation of an exposure level occurs, it takes time until a correct exposure is obtained and stabilized, and an awkward image is picked up during that period of time.
Further, in the image pickup system composed of the lens unit and the camera body according to the second example of prior art as shown in FIG. 23, there is such a second problem that, since the ND filter is tinged with some color, if the ND filter is inserted or detached when the 5600K mode or the 3200K mode is selected as the WB mode, the reproduction of color of an object image varies before and after the insertion or detachment of the ND filter.
In addition, since the image pickup apparatus according to the second example of prior art is of the interchangeable-lens type, when the lens unit is exchanged with another lens unit, ND filters change accordingly. In other words, the unevenness of the influence of the tinged ND filter makes it difficult to control the white balance mode on the side of the camera body.
Further, in the image pickup apparatus according to the third example of prior art as shown in FIG. 27, there is such a third problem, similar to the first problem, that, since the ND filter is mechanically inserted, when the ND filter is inserted in the process of an image pickup operation, an image plane becomes dark in that moment and, after a while, the iris control mechanism or the AGC control mechanism of the camera becomes operative, so that, when the same object image continues being picked up, a picked-up image gives an unnatural feeling to the user undesirably.